Bolts continue to be one of the most commonly used fasteners for removably securing a first member to a second member. They are relatively light in weight, provide a secure means of attachment and add shear load capability to the connection between the members to be joined together.
One problem with standard bolts, and bolted connections between two members, is that it can be difficult to manipulate the bolts during assembly and disassembly without dropping them. This is particularly true where the bolted connection is in a location which is difficult to reach and requires the use of a long wrench or a socket with a long extension. In some applications, such as the bolted connection between the fuel nozzle and engine case in a jet engine, as well as a variety of other bolted connections in jet engines, it is critical that the bolts not be dropped into the engine casing during their assembly and disassembly. Bolts which are mishandled during maintenance procedures and drop into the engine casing must be retrieved or catastrophic damage may occur to the engine during operation. This adds substantially to the time and effort involved in the maintenance procedure.
Fastener or bolt retainer assemblies have been proposed in the prior art to eliminate or at least reduce the chance of a bolt becoming disengaged from a first member as it is either assembled or disassembled from a second member. In many prior art designs, the shank of the bolt is formed with a groove which mates with a locking ring carried within a throughbore formed either in the first member or a retainer mounted to the first member. The bolt is inserted into the throughbore and through the locking ring so that its threaded portion mates with a correspondingly threaded bore in the second member. Upon removal of the bolt from the second member, the groove in the shank of the bolt engages the locking ring to prevent removal of the bolt from the first member or retainer. See, for example, U.S. Pat. No. 3,221,794 to Acres; U.S. Pat. Nos. 2,919,736 and 2,922,456 to Kann; and U.S. Pat. No. 3,217,774 to Pelochino.
One factor which complicates the design of a bolt retainer assembly where the first and second members are to be connected by a number of bolts is the "true position" tolerance of the bores in the second member to which the first member is mounted. The "true position" tolerance refers to the true or actual position of the bores formed in the second member compared to their position as designed and shown in assembly drawings for a particular part. The true or actual position of the bores varies from their design position within a certain tolerance, depending upon the part, and this tolerance must be accommodated by the bolt retainer in order to mount the first member to the second member. Usually, the true position tolerance is accommodated by making the bores in the retainer or first member larger than the bores in the second member so that the bolts can be shifted to a limited degree within the retainer or first member and aligned with the corresponding bores in the second member.
The true position tolerance is not accommodated in retainer assembly designs such as shown in the '794 patent to Acres. In this design, the bore in the first member is only slightly larger than the bore in the second member. A circular locking ring is movable with the shaft of the bolt and is carried within the bore of the first member. The bore in the first member of the Acres patent is too small to permit shifting of the bolt position to accommodate misalignment of the bore in the second member, and thus is only useful in application where a single bolt mounts the first and second members. If the bore in the first member of the Acres design was made larger in an attempt to adapt such structure for applications requiring a number of bolts, the circular locking ring could not be retained therein and would be ineffective to prevent disengagement of the bolt from the first member.
The problem of accommodating the true position tolerance of bores formed in the second member is recognized in the remaining patents cited above. The retainer assemblies disclosed in such patents each include a bore in a first member or retainer which is larger in diameter than the bore in the second member within which the bolt is threaded. The problem with these designs, however, is that the structure for retaining the bolt to the first member prior to assembly or after disassembly is relatively complicated, expensive and employs a number of different parts. Additionally, the parts must all be secured during flight or engine operation to avoid or reduce the destructive effects of vibratory stresses. The use of multiple parts to retain the bolt in place, which themselves might be lost during assembly and disassembly, is unacceptable and presents the same danger in jet engines as mishandled bolts.